D and A are definitely unique in terms of issues involved when it comes to racing, where C and B are definitely far closer in terms of why they are the way they are.
D you get people who might be under the impression they aren’t fit enough to race, or may lack the encouragement because of something along those lines (this isn’t always the case bear in mind, there are obviously lots of D’s who race, but… there’s probably far more who don’t)
A, I can definitely understand Tom’s point. Once you enter the realm of A, the sky is the limit of who you’re up against, and that’s… absolutely not encouraging to probably most people. You don’t have a clear understanding of who/what you’re against like you do in the lower categories; and that will also change day by day. There may some limited competition, the competition may be a cheater/uncalibrated trainer, or… it might be nobody at all.
D, C, B don’t have those open ends, you know you’re going into fairly limited (after the fact at least) capabilities (they’re broad, but… they don’t hold a stick to A; where for example someone like myself at 250w FTP can be penned against a 450w FTP monster at the same, or more, w/kg).
D however for some folks again I think is going to be more of a mental block than anything else. And if not a mental block, it could be a time limitation etc.
FRR I know is an insane thing to bring up in this context, but for the recent 3 week competition; a handful of 40ish Cat D riders completed the full tour… For the sake of comparison, it took those of us in the higher FRHC’s <25 hours for the full tour race time wise… meanwhile, some of the “D” based FRHC’s finished out well into the 40+ hours of racing (this isn’t even including warmup/cooldown times bear in mind).
But it’s worth considering for D’s, most of them will be on lighter loads / weekly (if even weekly), hours ridden; again most of this might be considered a mental block as well, but perhaps they’d rather ride to ride, and not “sweat it,” which is why they could be D’s.
And as we all know, C and B are definitely considerably larger fields with far more interest in racing (it’s also easier to get into, and most people end up getting into that competitive spirit not found in probably a mass majority of D riders on Zwift).
There’s definitely lots of reasons regardless…
Motivation across the board however, is pretty much the culprit.
The cause of lack of motivation is what varies.